**6. Conclusions**

88 Biogas

substrate utilization rate is expressed as a function of the organic loading rate by monomolecular kinetics for biofilm reactors such as rotating biological contactors and biological filters (Yu et al., 1998). This kinetic model can be used to describe carbonaceous removal in terms of BOD (biochemical oxygen demand), COD (chemical oxygen demand)

The original Stover-Kincannon model (Kincannon and Stover, 1982) (Equation 2) was initially proposed for rotating biological contactor (RBC) systems and can be expressed by

 d*S*/d*t* = [*Rmax* (*qSo*/*A*)]/[*KB*+(*qSo*/*A*)] (2) where: *A* is the disc surface area where the active biomass is attached; *S* is the substrate concentration in the reactor (in COD units) for a time (*t*); *So* is the initial substrate concentration; *q* is the flow rate; *Rmax* is the maximum removal rate constant and *KB* is the

In the modified Stover-Kincannon model the substrate utilization rate is expressed as

 d*S*/d*t* = [*Rmax* (*qSo*/*V*)]/[*KB*+(*qSo*/*V*)] (3) where *V* is the volume of the anaerobic reactor. The term d*S*/d*t* is defined for a steady-state

d*S*/d*t* = *q* (So – *S*)/*V* (4)

 V/[*q (So - S)*]= [*KB V*/*(Rmax q So)*]+[1*/Rmax*] (5) In continuously stirred tank reactors the hydraulic retention time (*HRT*) can be defined as:

 (*HRT*)/(*So* - *S*)= [*KB* (*HRT*)/(*Rmax So*)] + [1/*Rmax*] (6) According to this model a plot of (*HRT*)/(*So* - *S*) versus *HRT* should give a straight line of

As can be seen in Figure 4 the experimental data fitted to a straight line with R2= 0.9992 for OMSW 1 and R2= 0.9999 for OMSW 2. The maximum removal rate constant (*Rmax*) increased from 26.6 to 83.3 g COD/(L d) when the OMSW concentration changed from 35 to 150 g COD/L, indicating a good adaptation of the initial inoculum to the OMSW treated and to increasing concentrations of organic matter fed. The saturation value constants (*KB*) were 27.7 g COD/(L d) and 82.7 g COD/(L d) for OMSW 1 and OMSW 2, respectively. The values of *Rmax* and *KB* obtained for the concentrated OMSW were similar to those obtained by other authors for the anaerobic digestion of soybean wastewaters (Yu et al., 1998) and molasses (Büyükkamaci & Filibeli, 2002). Stover and Campana (2003) have shown that in the model *Rmax* is reduced by refractory organics and toxicity. Moreover, the refractory compounds change *KB* significantly from *Rmax*. These affirmations are in agreement with the data obtained in these experiments, where the higher organic concentration of OMSW 2 gave

and TOC (total organic carbon) as well as for nitrification.

function of the organic loading rate as follows (Yu et al., 1998).

saturation value constant (in g COD/(L d)).

relationship for different authors as:

Linearization of equation (3) gives:

*Rmax* values higher than for OMSW 1.

*HRT*=*V/q*, so equation (5) can be written as follows:

intercept [1/*Rmax*] and slope equal to *KB* /(*Rmax So*).

the following equation:

The kinetic constants obtained define the bio-treatability of the two-phase olive mill solid waste. The values obtained for *Rmax* and *KB* were similar to those obtained for other substrates of high organic content. The increase in the maximum methane removal rate for the most concentrated two-phase olive mill solid waste used demonstrated the good adaptation of the bacterial inoculum used to the increase in the substrate concentration. This adaptation allowed the microorganisms to work with high stability even with high organic matter concentrations in the fed substrate. These results can be used to estimate the treatment efficiency of industrial-scale reactors working with similar operational conditions.
